This invention relates generally to load responsive fluid control valves and to fluid power systems incorporating such valves which systems are supplied by a single variable displacement pump, equipped with an automatic load responsive control, and in which either one or a plurality of loads is individually controlled under positive and negative load conditions by separate control valves.
In more particular aspects this invention relates to direction and flow control valves and load responsive fluid power systems incorporating such valves capable of controlling simultaneously a number of loads under both positive and negative conditions.
Closed center load responsive central hydraulic systems are very desirable for a number of reasons. They permit load control with minimum of power losses and therefore, at high system efficiency and when controlling one load at a time provide a feature of flow control, irrespective of the variation in the magnitude of the load. In such a system a load responsive variable displacement pump control automatically maintains pump discharge pressure at a level higher, by constant pressure differential, than the pressure required to sustain the load. A variable orifice, introduced between pump and load, varys the flow supplied to the load, each orifice area corresponding to a different flow level which is maintained constant irrespective of variation in the magnitude of the load. The application of such a system is, however, limited by two basic system disadvantages. The pump control can maintain a constant pressure differential and therefore constant flow characteristics when operating only one load at a time. With two or more loads, simultaneously controlled, only the highest of the loads will retain the flow control characteristics, the speed of actuation of the lower loads varying with the change in magnitude of the highest load. This drawback can be overcome in part by the provision of a proportional valve as disclosed in my U.S. Pat. No. 3,470,694, dated Oct. 7, 1969. However, while this valve is effective in controlling positive loads it does not retain flow control characteristics when controlling negative loads, which instead of taking, supply the energy to the fluid system, and hence the speed of actuation of such a load in a negative load system will vary with the magnitude of the negative load. Especially with so-called overcenter loads where a positive load may become a negative load, such a valve will lose its speed control characteristics in the negative mode.
.Iadd.Negative load can be controlled by valve disclosed in U.S. Patent 3,534,774 issued to Tennis. However, this valve is capable of controlling negative loads in one direction only and therefore cannot be used with inertia type loads and loads driven by fluid motors which require bi-directional control capability. Further, the Tennis device does not isolate the load signal from the load controller. .Iaddend.